Data collection centring crystals

For high-throughput data collection, sample centring via loop detection is currently the method of choice for placing crystals in the X-ray beam. Alternative methods rely on the direct detection of the crystal itself by monitoring the intensity of either X-ray diffraction from the crystal or X-ray fluorescence from an element in the crystal that is not present in the cryoprotectant (Pohl et al., 2004). Both of these approaches have drawbacks. First, some of the limited X-ray lifetime of the crystal (Section 12.6) must be committed to detection instead of data collection. Second, the loop itself has to be placed close to the X-ray beam prior to crystal detection, followed by step-wise translation of the crystal through the beam, thereby increasing the time required to centre the sample. 

Fig. 11.6 Bi-dimensional X-ray diflxaction patterns of fibres of samples iPP3 with [rr] = 3.7 mol % (a-d) iPP4 with [rr] = 5.9 mol% (e-4) and iPP5 with [rr] =11.0 mol% (l-p), obtained by stretching at room temperature compressimi moulded films at the indicated values of the strain e. Data are collected in situ, at the beam-line 16.1 of the Synchrotron Radiation Souree in Daresbury (Cheshire, UK), 2.36 mm/min deformation rate, incident wavelength 2 = 1.4 A, 1 fiame/5 s. The stretching diiectirai is vertical. The strong reflections of the unoriented film specimens of iPP are used to determine the correct beam centre coordinates and the sample to detector distance. Arrows in b, f, g, m and n indicate the polarization of the reflection (040) of a form, (008).y of y form along the meridian, due to the perpendicular chain-axis orientation (cross p. Scheme 11.3a ) of crystals in y form occurring at intermediate deformations, before transformation into the mesophase at the onset of the fibrillar morphology...

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